Open Systems Interconnection (OSI) Model
Ever wondered how data flows through a network? How data comes from one device and arrives on another? Wonder no more, the Open Systems Interconnection Model, or simply, OSI Model, is here to explain it all.
The Open Systems Interconnection (OSI) model explains a structure of a network, or a networking framework to, implement certain protocols in each part called layers, with data being controlled and passed from one layer to another. The OSI Model is mainly used to understand the networking process and is considered a teaching tool nowadays. The OSI Model is one of the means used to inform devices when to send data and then not to transmit data. This is one of the methods which ensure that devices have a correct and error-free data flow rate.
The OSI model was created by the ISO or International Standard Organization to represent a standard architecture that will become the basis for network design. It is still being conceived by modern technologies. This happened in the mid-1980s, after years of having separate development processes on networking and devices were divided. With the help of the International Telegraph and Telephone Consultative Committee (CCITT), now called the International Telecommunications Union (ITU), The Open Systems Interconnection Reference Model was published.
There 7 architectural layers logically arranged in concept. From the lower layers that deals with physical signals and binary context, to the higher levels that handles user requests and application responses.
A common mnemonic used to remember the OSI model layers starting with the seventh layer (Application) is: “All People Seem to Need Data Processing.”, though, you can create your own mnemonic of the OSI Model.
Further below this post is an Analogy of the OSI Model or also known as the OSI Model Animation. Be sure to check it out as it explains a brief Train System Analogy of the OSI Model.
Layer 1 of 7 – The OSI Model
Physical Layer
Starting from the bottom of the receiving media, Layer 1 or the Physical layer of the Open Systems Interconnection Model is responsible for definitive and final transmission of digital data, called BITS, from the Physical layer of the sending device over network media to the Physical layer of the receiving device. This is known as a Source-Destination Process. Examples of Layer 1 technologies include Ethernet cables, hubs and other repeaters, and cable connectors.
The physical layer is an infrastructure that sits in a data center to provide the foundation for data centers. At the Physical layer, data are transmitted using the type of signaling that is supported by the physical medium: electric voltages, radio frequencies, wireless signals, or pulses of infrared or ordinary light. The bits, when received by the destination device, will pass the “physical data” on to the next layers.
Layer 2 of 7 – The OSI Model
Data Link Layer
When receiving data from the Physical layer, the Data Link layer is responsible for handling the physical transmission errors. It also packages bits into data called "frames", which is managed by physical addressing schemes such as MAC addresses for Ethernet networks, managing access controls of network devices to the physical medium.
The Data Link layer is a distinct and complex layer in the Open Systems Interconnection Model. Because of this, it is often divided into two parts, the "Media Access Control" sub layer and the "Logical Link Control" sub layer.
The data link layer of the OSI Model is also responsible for:
- Packet addressing
- Media access control
- Format the frame used to encapsulate data
- Error notification on the Physical layer
- Managing of error messaging specific to the delivery of packets.
- Ensures that frames are transmitted from one computer to another computer with no errors. It establishes error-free connections between two devices.
Layer 3 of 7 – The OSI Model
Network Layer
The Network layer handles routing and destination path finding above the Data Link layer. When data arrives at the Network layer, the source and destination addresses contained inside each frame are checked to determine if the data has reached its final destination and if the data is at the correct destination. If this is true, the frames are transformed into data packets and pushed to the next layers. The Network Layer is primarily responsible for establishing the paths used for transfer of data packets between devices on the network.
To achieve routing, the Network layer maintains logical addresses such as IP addresses for devices and machines on the network. The Network layer also manages the mapping between these logical and physical addresses. Router takes place in this layer.
Layer 4 of 7 – The OSI Model
Transport Layer
The Transport Layer ensures proper and error-free delivery of data across network connections. TCP is the most common example of a Transport Layer 4 network protocol. Different transport protocols may support a range of optional capabilities including error recovery, flow control, and support for re-transmission.
The Transport Layer is responsible for the delivery of messages between two or more networked hosts. It handles fragmentation and reassembly of messages and controls the reliability of a given link.
This includes:
- Guaranteed data delivery
- Name resolution
- Flow control
- Error detection
- Error recovery
Layer 5 of 7 – The OSI Model
Session Layer
The Session Layer manages the sequence and flow of events that initiate and tear down network connections. At Layer 5, it is built to support multiple types of connections that can be created dynamically and run over individual networks. This layer establishes process to process communications between two or more networked hosts. The important functions performed at Session layer to establish, maintain and terminate communication sessions
Layer 6 of 7 – The OSI Model
Presentation Layer
The Presentation layer is the simplest in function of any piece of the OSI model. At Layer 6, it handles syntax processing of message data such as format conversions and encryption / decryption needed to support the Application layer above it. It also defines the syntax that network hosts use to communicate.
The Presentation Layer of the OSI Model functions as protocol conversion, data translation, data encryption and decryption, Data compression, character set conversion, interpretation of graphics commands and much more.
Layer 7 of 7 – The OSI Model
Application Layer
The Application layer supplies network services to end-user applications. Network services are typically protocols that work with user's data. For example, in a Web browser application, the Application layer protocol HTTP packages the data needed to send and receive Web page content. This Layer 7 provides data to (and obtains data from) the Presentation layer.
Common application protocols include File Transfer Protocol (FTP), Telnet
Simple Mail Transfer Protocol (SMTP), Internet Message Access Protocol (IMAP), Post Office Protocol (POP), Hypertext Transfer Protocol (HTTP), Simple Network Management Protocol (SNMP) and the Network News Transfer Protocol (NNTP).
The OSI Model enhances teaching and learning of networking between devices and reduces the complexity of interoperability, which also creates a standard for all media. Unfortunately, as the technical and modern world advances to newer generations, actual application in implementation of the OSI Model is proving to be disadvantageous.
For more information, here is an in-depth tutorial of the OSI Model.
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